Higher-end headphones and headsets sometimes feature active noise cancellation (ANC). Headphones with ANC typically require two or more microphones and complex circuitry or processing to create suitable cancellation or compensation signals to be fed to the earphones. This often also requires battery power within the headphones or headset, which is inconvenient. Attempts to do ANC processing outside of the headphones or headset have typically involved digitization of the microphone signals, or customization of the audio jack (i.e. plug-and-socket) to add additional non-standard connectors in order to route the microphone signals. However, this adds extra cost or the need for special audio jacks that are not backwards compatible with existing equipment.
Similar reference numerals may have been used in different figures to denote similar components.